Method and apparatus for measuring intensity of magnetic fields



May 27, 1952 2,598,285

G. MUFFLY METHOD AND APPARATUS FOR MEASURING INTENSITY OF MAGNETICFIELDS Original Filed June 25, 1947 2 SHEETSSI-LEET l I E MEIR AXI soldm 1 7 p T Mfig gggmfirxx 5&5 500 EX ITIN AM LIPIBBR i m n mcmon oscIL/fiTOR I 'C ma'mac'roR fiMPl IFIER I 59 6O 65 64: AXI F IN T- 65" sit)!s: cuvrry MODULATION RfiJEQTING FILTitR RE R R +H.v. SUPPLY MODULATIONPHASE 7 FREQUENCY snn rmfi OSGLLLA'IQR FOfi 9 110 w DIFFERENCESYNCHRONIZING 119 CONTAC'IOR 16 t 56 4 57- c sisRma o exams To NE NE 55MBAJ AXIS IMBBL yams 'rwo PHB R 1 TWO PJHA E ER M TOR F R M I R 46 47 LP f INVENTOR. GARY M FFLY ms ATTORNEY I May 27, 1952 G M FFLY 2,598,285

METHOD AND AP PARATUS FOR MEASURING INTENSITY OF MAGNETIC FIELDSOriginal Filed June 23, 1947 2 SHEETSS!-IEET 2 GRID or GRID or GRID 0FGRID or p d I a GRID v Blfisgs col; 301w b ExcxmTmN ONE CYCLE OF NUTf-STI N W L J MW 9 .J Um

a} COIL 301G axcxcmcrloN H W f/V WMWM e INVENTOR. ARY M FFLY PatentedMay 27, 1952 METHOD AND APPARATUS FOR DIEASURING INTENSITY OF MAGNETICFIELDS Gary Muffly, Penn Township, Allegheny County, Pa., assignor toGulf Research & Development Company, Pittsburgh, Pa., .a corporation ofDelaware Original application June 23, 1947, Serial No. 756,426. Dividedand this application May 3, 1951, Serial No. 224,304

6 Claims. 1

This invention concerns a magnetometer for measuring the total intensityof a magnetic field. In particular, it concerns an improved form of"self-orienting total-intensity magnetometer suitable for use on amoving, tilting or gyrating vehicle for measuring the total intensity ofa magnetic field.

This application is a division of my copending application Serial No.756,426 entitled Apparatus for Measuring Intensity of Magnetic Field,filed June '23, 1947, now U. S. Patent No. 2,564,854, issued August 21,1951 and assigned to the same assignee as the present invention.v

Apparatus for measuring the intensity of a magnetic field for variouspurposes is well known. Many types of magnetometers suitable for specialpurposes are known in the prior art. The early magnetometers which wereused particul arly for measuring magnetic field intensity of the earthwere required to be set up on a fixed support and it was necessary toorient their axes of sensitivity manually in the direction of theparticular vector whose intensity was to be measured. In surveys of theearths magnetic field the direction of the total vector was not known inadvance and it was therefore customary to measure terrestrial componentsindependently, from which the total vector could afterwards be computed.

In copending application Serial No. 508,550, filed November 1, 1943 byVacquier and Muflly, now Patent No. 2,555,209, there is disclosed anapparatus which may be used for making magnetic surveys from a movingvehicle. One embodiment shown in application Serial No. 508,550 employsa detecting element of a type shown in Vacquier Patent No. 2,406,870mounted in a universal support and having its direction of orientationin space stabilized by means of auxiliary magnetically-sensitiveelements so that a desired component of the magnetic field may bemeasured as the magnetometer is moved about.

Another embodiment which is shown in application Serial No. 503,550employs a Vacquier detecting element mounted in a universal support withmeans for simultane'zously oscillating the magnetometer element abouttwo mutually-perpendicular axes in such a way that the detecting elementundergoes the magnetic field in various directions, these directionsforming the elements of a cone whose axis is the average direction ofthe magnetometer element. The normal output signa1 of the detectingelement is thereby modulated as a result of the directionaldisplacements and the signal is analyzed into its quadrature componentsof modulation, which quadrature components are applied to twoservomotorsrespectively so that the latter tend to orient the averageaxis of the magnetometer so as to reduce the modulation to a minimum.When the magnetometer is thus oriented, its signal output is a measureof the total magnetic vector and the device is a self-orientingtotal-vector magnetometer.

Copending application Serial No. 756,426, of which the presentapplication is a division, discloses and claims a magnetometer in whichthe sensitive element itself is not nutated but its instantaneous axisof sensitivity is caused to nutate by rotating about one end of thesensitive element an axially-asymmetrical magnetic vane. Theasymmetrical vane produces an angular displacement of the instantaneousaxis of sensitivity with respect to the geometric axis of the element,and the rotation of the vane brings about a revolution of theinstantaneous axis of sensitivity into various positions defined by acone, thus efiectively nutating the instantaneous axis of sensitivityabout the geometric axis.

The present invention is another form of the above-mentioned apparatusof the application Serial No. 756,426, simplifying the constructionthereof by the elimination of moving parts.

It is accordingly an object of this invention to provide a method andapparatus for measuring the total intensity of a magnetic field from amoving and tilting support.

Another object of this invention is to provide a method of sampling themagnetic field in various directions with a magnetically-responsiveelement and of utilizing the resulting signals for orienting the elementsubstantially in the direction of the magnetic vector.

Another object of this invention is to provide a magnetometer whichmeasures the total intensity of a magnetic field to a high degree ofprecision.

Another object of this invention is to provide an improved form ofmagnetometer which measures the total intensity of a magnetic field eventhough mounted on a moving and tilting support.

A further object of this invention is to provide a method by which amagnetometer signal is modulated in a manner from which any deviation ofthe direction of orientation of the magnetometer element from thedirection of the magnetic field may be corrected.

A further object of this invention is to provide apparatus whereby amagnetometer signal may be modulated in a manner so that the modulationsignal is a measure of the deviation of the magnetometer axis from thedirection of the magnetic field.

A further object of this invention is to provide apparatus whereby thesignal from a magnetometer is modulated in accordance with themagnetic-field intensity along directions adjacent that occupied by themagnetometer element and in which said modulation is used to orient thedetector in the direction of maximum magnetic intensity.

A further object of this invention is to provide a method and apparatusof modulating the signal from a directionally-sensitive magnetometer ina manner which will depend on the directional orientation of themagnetometer in the magnetic field, said modulation being accomplishedwithout moving the sensitive-detecting element itself.

These and other objects are attained in a manner which will be apparentfrom the following specification of which the accompanying drawings forma part, and in which Fig. 1 shows an embodiment of the invention,together with its schematic wiring diagram, in which displacement of theaxis of sensitivity of the magnetometer is accomplished without mov* ingthe sensitive element of the magnetometer; and

Fig. 2 shows excitation curves for the flux-deflecting vanes of Fig. 1.

The present invention concerns a magnetometer in which the instantaneousaxis of sensitivity is made to nutate about the geometrical axis of theelement but without the use of moving vanes. This invention isillustrated in Fig. 1. In the embodiment of Fig. 1 no moving vanes areemployed, and instead of a rotating vane a number of fixed vanes 30I-a,b, c and d are disposed in radial configuration about the detectingelement 300 which may be of the type shown and claimed in VacquierPatent 2,406,870. Four vanes with 90 spacing are shown in Fig. 1,although three vanes 120 apart or n vanes 360/n apart may be used if nis an integer greater than two.

Vanes 30l-a, 30l-b, 30l-c and 30|-d are mounted near the end of amagnetically-sensitive element 300 and symmetrically placed with respectto the principal axis of element 300. Each vane is wound with insulatedwire and may be excited through a transformer with A.-C. of a highfrequency, preferably of frequency far above the normal excitationfrequency of element 300. For example, if the element 300 is driven at1,000 cycles/sec, then the vanes 30l-a, 30l-b, 30l-c and 30I-d may, forexample, be excited at 30,000 cycles/sec. This high-frequency excitationshould be strong enough to saturate the vane with A.-C. flux and therebyreduce its effective permeability to the direct flux of the earthsfield.

Each vane 30I-a, 30l-b, 30l-c and 30l-d is in effect a magneticdeflector which allows some of the earths flux from its own side of thecore of detector element 300 to be detoured from its normal path. Ittakes some of the flux that would otherwise pass to one side of the coreof element 300 and leads it over to the core. If the vanes were allidentical and symmetrical, their total effect would be to increase thetotal fiux through core of element 300 without disturbing the axis ofsensitivity. If, however, any one vane conducts less than its share offlux, on the average, to the core of element 300, then the predominanceof the flux from the opposite side of the element causes the axis ofsensitivity to be inclined in the direction opposite to the vane. Thedetector 300 and its associated circuits are made to respond to theintegrated-magnetic effect, over periods of time long compared to theperiod of the high-frequency excitation of the vanes, but short comparedto the period of the nutation frequency. The high-frequency excitationshould be strong enough to saturate the vane with A.-C. flux so as toreduce its effective permeability during the time the high-frequencyexcitation is on. When any one of the vanes is so saturated, theeffective axis of sensitivity of the magnetometer is displaced in theopposite direction by the opposite vane or vanes, which are unsaturated.If each vane is saturated in turn, the axis of sensitivity can benutated continuously. This is accomplished with the circuit shown inFig. 1 as will be explained later. When the high frequency is notpresent, the value of flux in the vane depends on the ambient field andorientation and may assume any value between the saturation values inthe positive and negative senses. When the high-frequency excitation ispresent, however, the A.-C. excitation may be made strong enough tosaturate the vane over most of the cycle. Thus, the vane is saturated inone direction during one-half of the high-frequency cycle and issaturated in the opposite direction during the other half of thehigh-frequency cycle. Since the saturation fluxes are equal andopposite, their time aver age is zero over the whole cycle, and thus, ineffect, none of the ambient flux gets through. Of course, there will bea brief period when the flux is shifting from plus-to-minus saturationduring which time the ambient field can have some effect, but this canbe made as small as desired by increasing the high-frequency excitation.In practice, it is only necessary to increase the high-frequencyexcitation to the point where the ambient flux is sufficientlyrestricted to effect a displacement of the axis of sensitivity ofelement 300.

In Fig. 1 each vane, such as 30l-a, receives its saturating A.-C. fromhigh-frequency generator 304 by means of a commutating tube such as303-a. and transformer such as 302-11. The tubes are commutated, i. e.cut off and on, by means of a low-frequency multiphase generator 3635,having the same number of phases as there are vanes 30l. In theillustration, 4 phases are provided. A D.C. cut off bias may be providedby battery 306, or alternatively tubes 302 may be of a type that cutsoff at approximately zero bias. Transformers 302 serve to keep D.-C. outof the vane windings, as well as low-frequency transients which mightotherwise disturb operation of the device. In operation, thehigh-frequency generator 304 excites the vane 30! and the lowfrequencygenerator 305 controls the sequence of excitation by controlling thegrid bias on tubes 303.

Fig. 2 shows at 2-0. how the i-phase alternator 305 shifts theindividual grid biases of tubes 303 so that the tubes 303-a, b, c and dare rendered conducting in sequence. The superimposed high frequencyfrom source 304 is not shown in Fig. Z-a, but as each grid reaches itsmaximum upward swing, the high-frequency output of the tube reaches itspeak as shown in curves 2-!) to 2-e. Each tube 303 is active for abouthalf of the mutation period, the exact length being adjusted by properchoice of battery 305 and type of tubes 363 to make the -nutation followa smooth, approximately circular orbit. The use of three, four or morephases with symmetrical vanes is preferred. The response of detectorelement 308 should be fast enough to follow the frequency of generator305 but too sluggish to follow the high frequency of generator 364. Thedetector circuit connected to and following element 366 is normallyinherently sluggish toward the highest frequencies. This effect can beincreased by placing shielding of non-magnetic conductive material (notshown) around vanes 381-11, 1), c and d, or around element 3%, orbetween the vanes and the element. Any such shielding should beonly'heavy enough to be effective against the high frequency, as itshould have a minimum effect on the mutation frequency. If the aboverequirements are met, the magnetometer response integrates or averagesthe effect of vanes 30 i-a, b, c and d over several cycles of the high(saturating) frequency.

Fig. 1 shows the vanes 3M symmetrically disposed about one end of theelement-3B4]. This assembly may be mounted on a rigid, non-magnetic,electrically non-conducting framework (not shown) which in turn issuspended in gimbals (not shown) or other universal mountinginconventional manner. The orientation of the assembly 339-361 iscontrolled by servomotors '(not shown) which are mechanically connectedto rotate the assembly about the respective gimbal axes in well-knownmanner such as through the agency of belts or gears. These mechanicalconnections are schematically indicated in Fig. l by blocks i3 and 46,the servomotors themselves being schematically indicated by blocks 41and 48. Servomotors 41 and 48 are of the two-phase type. The operationof the motor-control and other circuits of Fig. 1 will now be described.

The detecting element sea, which maybe the type shown in Vacquier Patent2,406,870, is excited by an oscillator 53 as described in said patent.Electrical connections leading to and from the element 300, the groundreturn as well as any other connection which may be necessary to operateservornotors 4! and 48, may conveniently be carried by flexible leadsthrough hollow gimbals (not shown) or by means of brushes and slip rings(not shown) in well-known manner. As previously suggested, the frequencyof exciting oscillator 53 should be intermediate between the nutatingfrequency and the vane-saturating high frequency. A frequency of 1000 C.P. S. has been found satisfactory for oscillator 53.

The output signal from the sensitive element 3% passes through condenser59 to amplifier and demodulator 6G. The demodulator in unit 60 is ofconventional design and produces an output having a D.-C. component inproportion to the strength of the high-frequency output signal in thesame manner as a detector in a radio receiver produces a 11-0. componentusually employed for AVG. The demodulator 6E! also produces an A.-C.component which is proportional to the amplitude of modulation, also ina manner similar to that of a radio-receiver detector. The .D.-C.component of the demodulated signal output is transferred throughlow-pass filter 6| to recorder 62 which will be described later.Low-pass filter iii is for the purpose of rejecting the modulationsignal and keeping it out of the recorder. Circuit El is conventionaland may comprise two resistors and a condenser as is well known. TheA.-C. modulation signal output from the amplifier-demodulator 50 passesthrough the condenser 63 to the modulation amplifier 64 whose output is6 carried by connection 65 to both orienting servomotors 41 and 48.

A phasing J'signal may be obtained which "is related "to "the rotationof generator 305. This signal must bear a fixed phase relationship to thoutput of generator .3 8 5. This phasing signal isconv'enien'tlyobtained by means of a synchronizing contactor Ill) mechanicallyconnected to the shaft ofgenerator 305as indicated in Figure 1 byconnection '66. A small contact brush (not shown) makes contact with thesynchronizing contactor H0 upon each revolution of the generatorSBli. Inthis way the contactor H0 gives an indication of the phase position ofthe progressive excitation of vanes "30L The impulse from contactor H 0serves to synchronize an oscillator and associated amplifier indicatedby 56. Contactor I i0 closes a circuit each time the excita'tion ofelements 30! makes one revolution and the contactor also control thefrequency of the oscillator in unit '56. The unitSE comprisescomventional circuits which perform the functions of an oscillator andamplifier and are not shown in detail since theseare wellknown. Theoscillator may comprise a gas tube relaxation oscillator or amultiv'ibrator. Oscillators of this type may be made to lock in withtheimpulse applied through the contac'tor ,0 by adjusting the oscillatorto the approximate frequency of the contactor and applying the impulsefrom the contactor to the grid circuit of a .gas 'triode tube whichforms an element of the oscillator. The locked-in oscillator may befollowed bya tuned amplifier comprising one or more tuned stages ofamplification which will eliminate undesired harmonics.

The output of the oscillator-amplifier 56 may besp'litinto two phasesapproximately apart by means of a phase splitter H9. Any of the knowntypes of phase shifting networks may be used 'for this purpose. Onesimple and well known way of accomplishing such a phase shift is to use.a capacity of suitable value in series with the winding of oneservomotor 48. If it is desired tosh'ift the phase of both motorsimultaneously, this may be done by altering the tuning of the amplifier:in unit 55 by adjusting the capacity or inductance in one or more ofits tuned stages. The two quadrature outputs of the phase shifter H9 arefed by circuits 5! and 58 to the respective orienting servomotors 4! and48. Thus, the quadrature voltages in circuits 5! and 58 bear a fixedphase relation to the sequential excitation of the vane Bel-a, b, c andd.

The orienting servomotors 41 and 48 are convenien'tly of 'the'two-phaseA.-'C. type. One phase of each motor'i's energized continuously by A.-C.which is obtained by means of the contactor I I0, modulation-frequencyoscillator 56 and phase shifter 1 f9 and which is in synchronism withthe sequence of "excitation of vanes 39 I. If necessary this power maybe amplified up to a suitable level of a 'few watts and "adjusted to theproper phase. This power is supplied to the two motors 41 and 48 byvoltages whose phases differ by 90", that is the voltage in circuit 51supplying one phase of motor 111 is 90 out of phase from that in circuit53 supplying one phase of motor 48. The other phase of each motor isenergized in proportion to the output modulation of element 300 and issupplied through circuit 65 as previously explained. The signal incircuit 65 also follows the phase of the output modulation of element300. Now if a modulation is present in the magnetometer-"output signaland the two windings of either motor are excited in exactly the samephase, the motor will not run. This condition should obtain when thealignment error, i. e. misalignment between the axis of element 3M andthe direction of the ambient total magnetic vector, is at right anglesto the gimbal action of the motor in question. It may always be made soby inserting the proper amount of phase shift in a suitable place in thecircuit by conventional phase-shifting means. Under the same conditions,the other motor will be arranged to receive voltages differing 90 inphase so that it will run in the correct sense with an output limitedonly by the amount of modulation. An error of quite a few degrees in theservomotor supply phase adjustments has been found not to be serious, asthen the magnetometer orientation is restored along a curved or spiralpath which is not seriously longer than the ideal straight radial path.By way of example, a suitable orienting motor which may be used is theKollsman 60 cycle, model 776-02. Its 60 C. P. S. frequency represents areasonable rotational speed for the nutation and the motor is capable ofquickly stopping or reversing and requires only about 5 Watts per phase.The 60 C. P. S. frequency is also a convenient operating frequency forgenerator 305, generator 396 being of a much higher frequency, forexample 30,000 C. P. S. as suggested above.

In the operation of the apparatus, if the average axis of the sensitiveelement 36!] is in the direction of the total magnetic vector, itsoutput signal will have no modulation. This is because the angle whichthe axis of instantaneous sensitivity makes with the magnetic vector isalways the same at each point of the formers nutation.

Therefore, the output signal from element 3% will be a steadyunmodulated A.-C. whose value after amplification and demodulation(rectification) will be recorded by recorder 62. Since there will be nosignal in lead 65, the servomotors will not be actuated. Anymisplacement of the average axis of the magnetometer element with thedirection of total magnetic vector will cause a variation in the fieldundergone by element 300 during the cycle of mutation, resulting inmodulation of the output from the element 300, which in turn gives riseto a signal in lead 65. The phase of this signal in lead 65 willdetermine which of the motors ll and 48 will be caused to rotate andthus bring the magnetometer back to the normal unmodulated position.

The amplified signal representing the average output of the detectorelement 300 is recorded by recorder 62 which may be of theself-balancing potentiometric type. In order to increase the precisionof the reading on recorder 62 the greater part of the field beingmeasured by element 3M may be balanced out by a D.C. current supplied tothe output circuit of the magnetometer element 308' as described incopending application Serial No. 756,426 previously mentioned. The D.C.buck-out circuit does not alfect the A.-C. operation of the magnetometeras described above. When the ambient magnetic field is nearly balancedout in this manner, recorder 62 will indicate variations in theremaining portion experienced by element 300 and the precisionobtainable on recorder 62 may therefore be made very high.

What I claim as my invention is:

1. A self-orienting magnetometer comprising a magnetic field responsiveelement producing an electrical signal in proportion to the intensity ofmagnetic field along its principal axis of sensitivity, support means onwhich said element is rigidly mounted, a configuration of ferromagneticmaterial symmetrically placed around the axis of said element andrigidly mounted on said support, means for saturating respective partsof said material in progressive sequence around the axis of said elementwhereby the field affecting said element and the output of said elementis modulated in synchronism with said progression when the principalaxis of sensitivity of said element is not coincident with the directionof the ambient-magnetic field, a universal mounting for said support,orienting servo means mechanically connected respectively to the axes ofsaid universal mounting, and means electrically connected to saidelement responsive to quadrature components of modulation in the outputof said element controlling respectively said servo means in a manner torestore the principal axis of sensitivity of said element to coincidencewith the direction of the ambient-magnetic field.

2. A self-orienting magnetometer comprising a magnetic field responsiveelement producing an electrical signal in proportion to the intensity ofmagnetic field along its principal axis of sensitivity, support means onwhich said element is rigidly mounted, a plurality of ferromagneticvanes symmetrically placed around the axis of and near one end of saidelement and rigidly mounted on said support, means for saturating eachof said vanes in progressive sequence around the axis of said elementwhereby the field affecting said element and the output of said elementis modulated in synchronism with said progression when the principalaxis of sensitivity of said element is not coincident with the directionof the ambient-magnetic field, a universal mounting for said support,orienting servo means mechanically connected respectively to the axes ofsaid universal mounting, and means electrically connected to saidelement responsive to qua-drature components of modulation in the outputof said element controlling respectively said servo means in a manner torestore the principal axis of sensitivity of said element to coincidencewith the direction of the ambient-magnetic field.

3. A self-orienting magnetometer comprising a magnetic field responsiveelement producing an electrical signal in proportion to the intensity ofmagnetic field along its principal axis of sensitivity, support means onwhich said element is rigidly mounted, a plurality of ferromagneticvanes symmetrically placed around the axis of and near one end of saidelement and rigidly mounted on said support, means for applying asaturating alternating field to each of said vanes in progressivesequence around the axis of said element whereby the field affectingsaid element and the output of said element is modulated in synchronismwith said progression when the principal axis of sensitivity of saidelement is not coincident with the direction of the ambientmagneticfield, a universal mounting for said support, orienting servo meansmechanically connected respectively to the axes of said universalmounting, and means electrically connected to said element responsive toquadrature components of modulation in the output of said element controlling respectively said servo means in a manner to restore theprincipal axis of sensitivity of said element to coincidence with thedirection of the ambient-magnetic field.

4. A self-orienting magnetometer comprising a magnetic field responsiveelement producing an electrical signal in proportion to the intensity ofmagnetic field along it principal axis of sensitivity, support means onwhich said element is rigidly mounted, a plurality of ferromagneticvanes symmetrically placed around the axis of and near one end of saidelement and rigidly mounted on said support, electrical windings on eachof said vanes, means for exciting with an alternating current each ofsaid windings in progressive sequence around the axis of said element,said alternating-current excitation being of suflicient peak amplitudeto saturate each respective vane whereby the field affecting saidelement and the output of said element is modulated in synchronism withsaid progression when the principal axis of sensitivity of said elementi not coincident with the direction of the ambient-magnetic field, meansfor obtaining quadrature electrical signals which are related in phaseto said sequence of excitation, a universal mounting for said support,orienting servo means mechanically connected respectively to the axis ofsaid universal mounting, and means electrically connected to saidelement responsive to quadrature components of modulation in the outputof said element controlling respectively said servo means in a manner torestore the principal axis of sensitivity of said element to coincidencewith the direction of the ambient-magnetic field.

5. A method of periodically deflecting the axis of sensitivity of amagnetically-sensitive element which comprises placing adjacent to theelement a configuration of ferromagnetic material which is radiallysymmetrical with respect to the principal axis of sensitivity thereofand periodically saturating respective parts of said materialasymmetrically with respect to a plane perpendicularly bisecting saidelement and in progressive sequence around said axis.

6. A method of periodically deflecting the axis of sensitivity of amagnetically-sensitive element which comprises placing adjacent to theelement a configuration of ferromagnetic material which is radiallysymmetrical with respect to the principal axis of sensitivity thereofand periodically saturating respective parts of said materialasymmetrically with respect to a plane perpendicularly bisecting saidelement and in progressive sequence around said axis by applying to saidparts in said sequence an alternating field of frequency substantiallyhigher than the frequency of said sequence.

GARY MUFFLY.

REFERENCES CITED UNITED STATES PATENTS Name Date Hull Apr. 26, 1949Number

